Abstract: A rotary piston of a rotary piston internal combustion engine has radially and inner and outer annular seals carried within grooves of the rotary piston thrust surface axially facing the engine casing sidewall with the axial end face of the oil seal tapering radially outwards relative to the engine sidewall and forming lip angles between the tapered end wall of the seal such that the lip angle for the outer seal is between 1.degree. and 3.degree., the lip angle for the inner seal is between 3.degree. and 5.degree., and the spring biasing force acting on the radially outer seal is less than that acting on the radially inner seal. Preferably, an oil removal bore is formed in the land portion between the grooves for return of the oil delivered thereto by action of the oil seal to the oil pan of the engine.